Particular embodiments generally relate to electro-magnetic interference reduction systems.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
An audio amplifier may be used in a portable application because of the audio amplifier's high efficiency of power delivery to a load, such as a headphone and/or speakers. However, class-D audio amplifiers may produce electro-magnetic interference (EMI). A class-D amplifier generates a pulse-width modulation (PWM) signal that is based on an input audio signal. The PWM switching frequency is in the range of several hundreds of Kilohertz (KHz). The audio amplifier may be used in a portable device, such as a cellular phone, tablet, or smart phone. The PWM carrier frequency and/or its harmonics may reside in a radio frequency range and causes electro-magnetic interference to radio frequency signals in the portable device. Different methods have been used to reduce electro-magnetic interference. For example, inductor capacitor (LC) filters, shielding, ferrite beads, and spread spectrum modulation have been used.
Inductor-Capacitor (LC) filters may be used at the amplifier output. However, the LC filters are large and expensive, which increases the system cost.
Shielding may be used to cover the EMI emissions. For example, printed circuit board (PCB) traces that carry high frequency signals may be routed between ground planes, which partially cover the electro-magnetic signals with ground conductive shields. However, the shields increase system weight, cost, and use additional area on the PCB.
Ferrite beads are passive electronic components that are used to suppress high frequency noise. Ferrite beads act as a radio frequency (RF) choke and placing the ferrite beads in series with the load (e.g., speakers) close to the audio amplifier may attenuate high frequency signal components and reduce EMI. However, the ferrite beads are effective over a narrow frequency range and may not provide enough attenuation over the output noise bandwidth.
Spread spectrum modulation may be used where the switching frequency of the PWM signal is fluctuated around a center switching frequency. This spreads the energy centered in the switching frequency and its harmonics to neighboring frequencies. Various frequency fluctuations exist, such as random and chaotic modulation. However, these methods do not attenuate the EMI significantly and also degrade the audio in-band noise floor (e.g., degrade the signal to noise ratio (SNR)).
A permissible level of conductive and radiated EMI by any device is regulated by a number of governing bodies throughout the world to ensure electro-magnetic compatibility (EMC) of all electronic equipment. The Federal Communications Commission (FCC) includes specifications for radiated EMI for two different categories of devices, Class A and Class B devices. Class A devices are categorized as business/industrial/commercial use devices, whereas Class B devices are categorized as residential use devices. Table 1 shows an example of the limits for Class A and Class B devices.
TABLE 1RadiatedRadiatedEmissionsEmissionsFrequencyLimits for ClassALimits for ClassBRange(dB uV/meter(dB uV/meterMHz@ 10 meters)@ 3 meters)FCC Radiated[30 88]39.140EMI Limits for [88 216]43.543.5Class A and[216 960]46.446.0Class B products [960 10000]49.554.0Also, FIG. 1 shows the limits for radiated EMI for the class B category.
A class-D amplifier uses a fixed frequency PWM signal. Large tones are observed at the PWM carrier frequency and its harmonics. The tones are illustrated in FIGS. 2, 3, and 4. The following simulation set-up and results are used to show the tones that may be observed:
Simulation Setup—
Input signal sample rate48 KHzSignal fundamental frequency937.5 HzSupply Voltage1.8 vPWM switching frequency5.080320092962877e+05 HzPLL Clock1.625702400000000e+09 HzRBW104 HzResults—
SNR  121.9556 dBTHD + N−116.2588 dB (0.00015%)Min attenuation in FCC band1   41.517 dB30 MHz-300 MHzMin attenuation in FCC band2   54.426 dB300 MHz-1 GHzIn FIG. 2, a graph 202 shows the PWM output. Also, a graph 204 shows the PWM output around the PWM frequency. A graph 206 shows tones in a FCC band1 (30 MHz-300 MHz) and a graph 208 shows tones in a FCC band2 (300 MHz-1 GHz). Tones near the frequency 30 MHz are as high as −41.5 dB and tones near the frequency 300 MHz are around −54.4 dB. Signal to Noise ratio (SNR) is 121.96 dB and total harmonic distortion+noise (THD+N) is −116.26.
Referring to FIG. 3, a graph 302 shows tones in a start region of FCC band1, and a graph 304 shows tones in a start region of FCC band2. As shown, the tones in FCC band1 are around −41.5 dB and the tones in FCC band2 are around −54.4 dB. FIG. 4 shows tones around the end-region of FCC band1 and band2. In a graph 402, the tones near the frequency 300 MHz are around −55 dB and in a graph 404, the tones near the frequency 1 GHz are around −65 dB. The PWM tones observed in FCC band1 and band2 are an interference source in the RF domain.